The present invention relates to capacitance multiplier circuits and, more particularly, to circuits in which the output capacitance has a variable capacitive reactance value that can be electronically varied over a predetermined range of values.
Capacitance multiplier circuits are well known in the art. For example, U.S. Pat. No. 3,911,296 discloses an integrated circuit which has an effective capacitance that is equal to the capacitance of a capacitor multiplied by a factor of beta, where beta is the common emitter current gain of an NPN transistor formed in an integrated circuit including the capacitor. The capacitor is coupled between the collector and base of the transistor. If an alternating current (AC) is introduced at the collector of the transistor a proportion thereof flows into the capacitor and into the base of the transistor to produce a current flow at the collector thereof that is equal to beta times the base current. The total current flowing into the circuit node to which the collector and capacitor are interconnected is equal to a value of a sum of the two currents. Thus, the effective capacitance value seen at the circuit node is greater than the capacitance value of the capacitor by a factor of beta.
There are many applications for electronically tunable reactance circuits. For instance, frequency modulator (FM) systems often use a crystal controlled oscillator circuit that includes a variable capacitance for frequency modulation. For example, one prior art technique that has been used is to provide a variable capacitance diode in combination with the aforementioned oscillator to provde direct frequency modulation. However, it is difficult or impossible to realize a variable capacitance diode in an integrated circuit such that the prior art is not useful for integrated frequency modulation schemes. Instead, some prior art integrated circuits such as the MC1376, manufactured by Motorola, Inc., have provided frequency modulation by changing the feedback phase of the oscillator instead of using a variable capacitance diode.
Therefore, if the effective capacitance of the above described capacitance multiplier circuit could be varied over a predetermined range such a circuit could be utilized in an integrated circuit for frequency modulation.
Thus, a need exists for a capacitance multiplier circuit of which the capacitive reactance can be electronically varied over a predetermined range of values and which is suited to be manufactured in integrated circuit form.